1. Field of the Invention
The present invention relates to the field of brushless DC motors, and more particularily to a method for starting sensorless brushless DC motors.
2. Background Art
In a brushless DC motor, commutation is provided by selectively applying DC current to the motor's field windings. Whether and in what direction current is applied to a particular winding depends on the physical and operational configuration of the motor and the rotational position of the motor's armature. Each motor has a discrete number of commutation states, depending on the number of windings or phases of the motor and the motor's mode of operation (e.g. unipolar, bi-polar, etc.). A three-phase motor in unipolar operation has three different commutation states. A three-phase motor in bi-polar operation has six different commutation states. The commutation states for a three-phase, bipolar motor are shown in Table 1.
TABLE 1 ______________________________________ COMMUTATION STATES FOR THREE-PHASE BIPOLAR DC MOTOR Commutation State Winding 1 Winding 2 Winding 3 ______________________________________ 1 Positive Negative Open 2 Positive Open Negative 3 Open Positive Negative 4 Negative Positive Open 5 Negative Open Positive 6 Open Negative Positive and repeating: 1 Positive Negative Open ______________________________________
where:
Positive means that a positive potential is applied to the indicated winding. PA1 Negative means that a negative potential is applied to the indicated winding. PA1 Open means that no potential is applied to the indicated winding PA1 THETA=angular displacement in radians PA1 PI=3.14159265 PA1 POLES=number of poles on armature PA1 PHASES=number of phases
For continuous operation of the motor, the commutation applied to the motor must be advanced to successive commutation states as the motor rotates. Each commutation state is applicable over a finite angular displacement of the motor's armature. This finite angular displacement can be calculated from the formula: EQU THETA=(2*PI)/(POLES*PHASES)
where:
For a three-phase motor having a 12-pole armature, the above equation results in a value for THETA of 0.1745 radians or 10 degrees. Anotherwords, each commutation state applies for 1/36th of a revolution. The applied commutation must be advanced to the succeeding commutation state after every 10 degrees of the armature's rotation. For one complete revolution, 36 successive commutation states must be applied, corresponding to six cycles through the six commutation states listed on Table 1.
For the proper timing of commutation advancement, a means for determining the angular position of the motor's armature is required. In sensored brushless DC motors of the prior art, optical or hall effect position sensors are used to determine the angular position of the motor's armature. In sensorless brushless DC motors, however, the angular position of the armature is not measured directly. Instead, it is deduced from a characteristic polarity reversal that occurs in back EMF induced by the rotation of the armature in the motor's undriven field coil windings. Sensorless brushless DC motors have the advantage of simplicity and reduced cost since the need for separate angular position sensors is eliminated. However, the armature must be rotating at a significant angular velocity before sufficient back EMF is produced to allow back EMF sensing. It has heretofore been difficult to provide effective commutation of a sensorless brushless DC motor during the motor's start up period when the motor is spinning below this speed.
One method that has been used in the prior art is generating a fixed frequency timing pulse that triggers successive commutation states. The frequency of the timing pulse must be greater than the frequency at which commutation states advance at a rotational speed of the motor at which sufficient back EMF is generated to allow effective back EMF sensing.
This starting method has several drawbacks. Upon initial start-up, the timing pulses advance the applied commutation states more quickly than the angular position of the armature requires. As a result of this incorrect timing, the current supplied to the motor's field windings by the successively applied commutation states at times will oppose the motor's rotation, causing erratic and inefficient rotational accelleration of the motor and long start-up times. In the worst case, reverse rotation of the motor can occur.